DC-DC converters that convert a variable DC input voltage to a DC voltage of a predetermined electric potential and outputs the converted DC voltage include step-up/down DC-DC converters that can step up or down an input voltage and output the converted DC voltage. Inventions on such conventional step-up/down DC-DC converters are described in, for example, Patent Documents 1 and 2.
FIG. 8 illustrates the configuration of a step-up/down DC-DC converter disclosed in Patent Document 1. This step-up/down DC-DC converter has a switching device S1, an inductor (coil) L, and a rectifying diode D2 that are connected in series between an input terminal IN receiving a DC voltage Vin supplied from a DC source, such as a battery, and an output terminal OUT. A diode D1 is inversely connected to an input terminal (node N1) of the inductor L and a grounding point, and a switching device S2 is connected to an output terminal (node N2) of the inductor L and a grounding point.
As illustrated in FIG. 9, a control circuit that turns on/off the switching devices S1 and S2 includes a triangle-wave generator circuit TWG that generates triangle waves used for PWM (pulse width modulation) control of the output voltage, an inverting amplifier AMP that inverts a feedback voltage FB1, and paired PWM comparators CMP1 and CMP2 that receive the triangle waves and the feedback voltage FB1 or an inverted voltage FB2 thereof. If the input voltage Vin is higher than a target output voltage, i.e., if the feedback voltage FB1 is lower than the peak voltage V1 of the triangle waves, the switching device S2 enters a continuous OFF state while the switching device S1 is driven by PWM pulses to output a voltage Vout that is obtained by stepping down the Vin. If the input voltage Vin is lower than the target output voltage, i.e., if the feedback voltage FB1 is higher than the peak voltage V1 of the triangle waves, the switching device S1 enters a continuous ON state while the switching device S2 is driven by PWM pulses to output a voltage Vout that is obtained by stepping up the Vin.